Great frigatebirds, Fregata minor, choose mates that are genetically similar

Inbreeding occurs infrequently in most natural populations, with the level of relatedness between mates potentially being influenced by behavioural mechanisms of mate choice and spatial genetic structure of the population. We studied mate relatedness in a Hawaiian population of great frigatebirds with the expectation, based on frigatebird ecology, that mated pairs were unlikely to be more genetically similar than a random draw of adults from the breeding population. We used a paired analysis to compare the DNA fingerprint similarity of a female and her mate versus the same female and a random male that was breeding elsewhere on the same island. Overall, band-sharing scores between mates were higher than band-sharing scores between nonmates, and the mean coefficient of relatedness between mates was 0.082. Relatedness between mates was not a consequence of strong natal philopatry coupled with random mate choice: historical data revealed several recent between-island shifts in the location of the breeding colony, and the degree of genetic similarity between nonmates was not well predicted by the physical distance between their nest sites on Tern Island. Instead, females may deliberately choose related individuals as mates. Tests for a relationship between the genetic similarity of mates and the size and ectoparasite load of their 1-week-old chick were equivocal. The question of how and why females may be choosing genetically similar mates is unresolved, but it appears to be a consequence of active choice rather than spatial genetic structure or limited availability of unrelated mates. (C) 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.